1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a backlight assembly and an LCD device using the same.
2. Discussion of the Related Art
The cathode ray tube (CRT) has been widely used in television receivers and in monitors of measuring instruments, information terminals, and the like. However, the size and weight of a CRT limits production of small, lightweight CRT-containing devices. As a result, other display devices are increasingly taking the place of CRTs. Examples of these display devices include liquid crystal display (LCD) devices utilizing an electro-optical effect, plasma display panels (PDP) utilizing gaseous discharge, and electroluminescent display (ELD) devices utilizing electro-luminescence.
In view of their light weight, thin profile, and low power consumption, LCD devices are among the most popular display devices. Accordingly, there is a great demand for LCD devices, which are widely utilized in desktop computers, laptop computers, and large-size display devices.
A typical LCD device includes an LCD panel for displaying a picture image and a driving part for applying a driving signal to the LCD panel. The LCD panel typically includes first and second glass substrates and a liquid crystal layer interposed therebetween. Spacers maintain a predetermined gap enabling the first and second glass substrates to be bonded by a sealant having a liquid crystal injection inlet through which liquid crystal is injected.
The first glass substrate (TFT array substrate), includes a plurality of gate and data lines, a plurality of pixel regions and pixel electrodes, and a plurality of thin film transistors (TFTs). The plurality of gate lines runs in one direction at fixed intervals. The plurality of data lines run perpendicular to the gate lines at fixed intervals. A matrix of pixel regions is defined by the plurality of gate lines and data lines crossing each other in which each pixel region contains a pixel electrode. Each of the TFTs is arranged to be switchably responsive to a signal from a selective gate line, which is transmitted through a data line to a pixel electrode.
The second glass substrate (color filter substrate) includes a black matrix layer for shielding light incident on regions excluding the pixel regions of the first substrate, a color filter layer (R, G, B) for displaying colors, and a common electrode for obtaining a picture image.
An LCD device displays picture images by controlling transmittance of ambient light. However, because an LCD device does not emit light, the LCD device requires an additional light source, such as a backlight unit. A backlight unit may be classified as a direct-type or edge-type unit depending on the position of the light source relative to the display area.
An LCD device may employ a variety of light sources in a backlight unit, including electroluminescent (EL) lamps, light-emitting diode (LED) lamps, cold cathode fluorescent lamps (CCFL), and hot cathode fluorescent lamps (HCFL). Because of their long lifetime, thin profile and low power consumption, CCFLs are often used as light sources for large-sized color TFT LCD devices.
The fluorescent discharge tube used in a CCFL utilizes the penning effect when injecting a hydrargyrum gas containing Argon Ar and Neon Ne at a low temperature. Electrodes are formed at both ends of the fluorescent discharge tube, and a plate-shaped cathode is formed therein. When a voltage is applied thereto, electric charges inside the fluorescent discharge tube collide against the plate-shaped cathode, generating secondary electrons. Circumferential elements are excited by the secondary electrons and plasma is generated. In the process, the circumferential elements emit strong ultraviolet rays, which excite a fluorescent substance, thereby emitting visible rays.
In an edge-type backlight unit, a lamp unit is installed at one side of a light-guiding plate. Edge-type backlight units are typically utilized in smaller LCD devices, such as laptop or desktop computer monitors. An edge-type backlight has a thin profile and can provide uniform luminance and a long life in an LCD device.
Direct-type backlight units are being increasingly developed for use in large LCD devices that are 20-inch or larger in size. A direct-type backlight unit includes a plurality of lamps formed in a line on a lower surface of a light-diffusion plate, whereby the entire surface of the LCD panel is directly illuminated with the light. A direct-type backlight unit utilizes light more efficiently than an edge-type backlight unit. Since larger LCD devices require higher luminance, direct-type backlight units are often utilized in larger television monitors.
However, because monitors employing a direct type backlight unit employ a plurality of lamps and are used for long periods of time, the lamps are prone to fail. In contrast, an edge-type backlight unit having lamp units at both sides of the light-guiding plate is less affected by a failed lamp unit, because the decreased luminance accompanying a failed lamp does not present as much a problem as compared to an LCD device employing a direct-type backlight unit.
An LCD device carrying a direct-type backlight unit includes a plurality of lamps underneath a screen. If one of the lamps fails or reaches the end of its lamp life, a portion of the screen corresponding to the location of the failed lamp becomes darker in comparison to the surrounding portions of the screen. Accordingly, there is a need for direct-type LCD devices having simple structures suitable for disassembling and re-assembling a backlight unit assembly in an LCD device.
FIG. 1 is a perspective view depicting a direct type backlight assembly according to the related art. The direct type backlight assembly includes a plurality of fluorescent lamps 1, an outer case 3, and light-scattering means 5a, 5b and 5c disposed between the fluorescent lamps 1 and an LCD panel (not shown). A fluorescent substance is coated on an inner surface of each fluorescent lamp 1. The outer case 3 fixes and supports the plurality of fluorescent lamps 1.
The light-scattering means 5a, 5b and 5c prevent a silhouette of the fluorescent lamps 1 from appearing on the surface of the LCD panel screen, and provide a light source having uniform luminance. To increase light-scattering of non-uniform light from the fluorescent lamps, the direct-type LCD device includes a plurality of diffusion sheets, a diffusion plate and a protective sheet between the fluorescent lamps 1 and the LCD panel.
However, backlight assemblies employing fluorescent lamps according to the related art may often utilize fluorescent lamps having emission characteristics producing a low color realization ratio. Moreover, the ability to obtain a backlight unit having high luminance is hampered by limits in size and capacity of the fluorescent lamp.
Backlight units are often used for illuminating the screen of an LCD device, whereby the viewer can read information displayed on the screen in dark surroundings. To satisfy the demand for LCD devices having suitable design characteristics, a thin profile, and low power consumption, there is a increasing need for backlight units utilizing a thin light-guiding plate, a function for displaying various colors, and LED lamps requiring less power consumption.
FIG. 2 is a perspective view depicting a backlight assembly employing LED lamps according to the related art. In FIG. 2, LED light sources 22 are positioned at both sides of a light-guiding plate 21 formed at the rear of an LCD panel. The LCD panel is illuminated with light emitted from the LED light sources 22, thereby enabling the display of images in dark surroundings. Each of the LED light sources 22 includes a plurality of lamps 23 arranged in one direction at fixed intervals, wherein the lamps 23 include red, green and blue LEDs.
When displaying images on the LCD panel, the backlight unit turns on the LED lamps 23 in the LED light sources 22. As a voltage is applied to the LED lamps 23, light is emitted and dispersed in the light-guiding plate 21. The dispersed light is color-mixed, whereby the LCD panel is illuminated with white-color light. The use of LED lamps in the backlight unit of an LCD device facilitates low power consumption and miniaturization.
An LED is a solid, semiconductor photoelectric conversion device providing higher reliability and a longer life than a CRT. To emit light from an LED, a D.C. voltage of 1.5V is applied thereto. Because there is no requirement for a DA-AC converter, LEDs facilitate decreased power consumption.
However, related art LED backlight assemblies possess certain disadvantages. For example, when LED light sources are positioned at opposite sides of the light-guiding plate, it is difficult to generate uniform white-color light by color-mixing the red, green and blue light emitted from the red, green and blue LEDs. Accordingly, this may lead to reduced lighting efficiencies and color realization ratios.